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Toxic blue-green algae at Rietvlei - 9 March 2007
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Friday’s scorching temperatures have resulted in the Rietvlei Wetland Reserve having to remain closed this weekend, due to a health risk caused by a toxic blue-green algae, which thrives in hot weather.  The blue-green algal bloom poses a health risk to recreational users of the water body. The reserve is an important ecological area and is well used by various recreational groups.

Cliff Dorse, of the City’s Biodiversity Management Branch, says the unicellular algae combine into small colonies and these can be seen as little green specks in the water column. The algae wash up on the shore line and collect in the shallows, depending on which way the wind is blowing.

“In such areas it can become so dense that it forms a thick layer of yellowish-green scum. It is in such areas that the toxin concentrations are potentially at their highest,” says Dorse.

The presence of the algae was identified last Friday and a monitoring programme was initiated. The results from yesterday (Thursday) morning’s sample appeared very promising and it looked likely that the vlei would be open for the weekend.

“Unfortunately Friday’s very hot weather was perfectly suited to the exponential growth of the algae. This resulted in an explosion in algal activity and numbers and the decision was made to keep the water body closed this weekend,” says Dorse.

He added this was a precautionary measure and the vlei would be re-opened as soon as further laboratory analysis had shown that the water was fit for use. Additional water samples will be collected throughout next week in an effort to get a clear understanding of the fluctuations of the algae. A large number of City departments, including Environmental Health, Roads and Stormwater and Scientific Services have been involved in advising the City’s Biodiversity Management Branch on what actions to take.
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